A conventional transmission of the above type includes: a power transmission system having a plurality of speed gears (e.g. forward first to seventh speed gears and reverse first and second speed gears); and a lubricating oil amount reducing mechanism for reducing the amount of lubricating oil supplied to the power transmission system. When the present speed gear of this transmission is a specified speed gear of the plurality of speed gears (for example, the forward sixth or seventh speed gear which involves a relatively small load), the lubricating oil amount reducing mechanism is activated to reduce the amount of lubricating oil, thereby reducing loss of horsepower.
However, this conventional transmission has the following problem. When the present speed gear is the above-mentioned specified speed gear, a shift is performed with the amount of lubricating oil reduced by the lubricating oil amount reducing mechanism, so that the power transmission system runs short of the lubricating oil at the time of the shift and thus may result in reduced durability of the power transmission system.
The present invention is directed toward solving such a problem and aims to provide a transmission capable of: reducing loss of horsepower by reducing the amount of lubricating oil supplied to a power transmission system when the present speed gear is a specified speed gear; and securing the amount of lubricating oil for the power transmission system at the time of a shift performed when the present speed gear is the specified speed gear.